1. Field of the Invention
The present invention relates to a scanning module for imaging through scattering media. More particularly, the present invention is directed to a scanning module for imaging through scattering media while alleviating adverse effects of the weak transmission through highly scattering media. According to the present invention the injection of photons is optimized so that the overall transmission is increased compared to the conventional art. In addition, the present invention eliminates cross-talk effects in a multi-port geometry thereby increasing parallelism.
2. Background of the Invention
Infrared imaging through scattering media is an area of research that has created enormous interest. A main target application is breast cancer detection, in which the use of a near infrared short-pulse laser is particularly appropriate. Obtaining images of the interior of the breast is complicated by the extensive scattering of light in such a medium. The scattering property of the breast causes a weak transmission through a thick slab and blurring of the image obtained from the transmitted light. The use of ultra-short laser pulses combined with an ultra-fast and ultra-sensitive detection system reduces the adverse effects of scattering.
The trajectory of a photon propagating inside a scattering medium can be predicted only on a statistical basis. In addition to the probability of being absorbed, the photons are subject to numerous scattering events. In a slab medium that is highly scattering and weakly absorbing, such as the human breast, most photons are reflected toward the entrance surface after traveling only a few millimeters into the tissue. Other photons are absorbed by the medium or transmitted to the output surface where they can be detected. In the case of typical breast thickness and optical parameters, 0.001 to 1% of the injected photons are transmitted to the output surface.
The light transmitted to the output surface carry the most significant information about the interior of the medium, since the reflected light carry information predominantly from a region close to the input surface. In previous systems as shown in FIG. 1, images of the interior of a scattering medium 24, such as the human breast, have been obtained by scanning a laser beam 20 of small diameter along the input surface 22 and detecting light emerging from a small area along the output surface 26. The grey region 25 indicated in the dotted line in FIG. 1 illustrates the region through which the photons pass when traveling from the injected point to detector 29. As discussed previously, such an imaging technique is complicated by the strong scattering property of the medium: the number of transmitted photons is very weak compared to the number of injected photons and the scattering causes a strong blurring of the image. There is a need for developing techniques that alleviate the blurring and the weak transmission associated with the scattering.
Referring to FIG. 1, when a single photon is injected at the input surface 22 of a scattering slab 24, it has a certain probability to be transmitted to the opposite surface 26 at some point and after some traveling time. Similarly, when injecting an infinitely short light pulse at the input surface 22, a certain proportion of light (typically weak) is transmitted. The transmitted light can then be collected over a certain spatial region of the output surface and during a certain time interval. In other words, when arriving at the output surface 26, the input pulse is spread spatially and temporally forming a spatio-temporal distribution of the transmitted light.
Theoretical expressions (the diffusion model) allow to correctly predict the spatio-temporal distribution of the emerging light from the output surface corresponding to the injection of an ultra-short light pulse at the input surface of a homogeneous scattering slab. FIGS. 2 and 3 illustrate examples of the theoretical temporal and spatial distributions of the transmitted light in the case of a 40 mm thick scattering slab having a scattering coefficient .mu..sub.s '=1 mm.sup.-1, an absorption coefficient .mu..sub.a =0.002 mm.sup.-1 and a refractive index n=1.34. The scattering coefficient .mu..sub.s ' is the probability of the photon being isotropically scattered per infinitesimal pathlength, and the absorption coefficient .mu..sub.a is the probability of the photon being absorbed per infinitesimal pathlength. FIG. 2 shows the temporal distribution of the spatially-integrated transmission, and FIG. 3 shows the spatial distribution of the time-integrated transmission. In FIG. 2, the time origin corresponds to the entry of the light pulse in the medium. In FIG. 3, the radial position 0 corresponds to the point of the output surface facing the injection point of the input surface. As can be seen in this particular case, the part of the input pulse that reaches the output surface emerges into a 40 mm Gaussian spot temporally spread during a time interval of about 5 ns.
Similar to the transmission mechanism, when a single photon is injected at the input surface of a scattering slab, it has a certain probability of being reflected back to the input surface at some point and after some travel time. When injecting an infinitely short pulse at the input surface, a certain proportion of light (typically very high) is reflected. The reflected light can then be collected over a certain spatial region of the input surface and during a certain time interval. In other words, the reflected pulse is spread spatially and temporally forming a spatio-temporal distribution of the reflected light.
Again, the diffusion model provides theoretical expressions for the spatio-temporal distribution of the emerging light from the input surface corresponding to the injection of an ultra-short light pulse at the input surface of a homogeneous scattering slab. FIGS. 4 and 5 show examples of the theoretical temporal and spatial distributions of the reflected light in the case of a 40 mm thick scattering slab having a scattering coefficient .mu..sub.s '=1 mm.sup.-1, an absorption coefficient .mu..sub.a =0.002 mm.sup.-1 and a refractive index n=1.34. FIG. 4 shows the temporal distribution of the spatially-integrated reflection and FIG. 5 shows the spatial distribution of the time-integrated reflection. In FIG. 4, the time origin corresponds to the entry of the light pulse in the medium, and in FIG. 5, the radial position 0 corresponds to the injection point. As can be seen for this particular case, the part of the input pulse that is reflected back emerges into a 2 mm Gaussian spot temporally spread during a time interval of about 50 ps.
The scanning process illustrated in FIG. 1 is typically slow due to the weak number of transmitted photons and the typically required averaging. One way to achieve a faster scan is to use parallelism, that is a duplication of the scanning process in many ports 60, as illustrated in FIG. 6. Each port 60 performs a scan over a reduced region allowing for a faster process. As far as possible, the different ports 60 must be independent and thus the spacing between two adjacent ports must be sufficiently large to avoid cross-talk effects. The cross-talk (interaction between ports) is superimposed on the useful signal (the light traveling from the input to the output of a single port). The larger the port spacing, the weaker the cross-talk will be, but the larger the scanned region for each port 60. As a result, there is a trade-off between the cross-talk and the scanning speed. For the particular case of FIG. 3, to avoid significant cross-talk, the port spacing must be about 40 mm. In general cases, the port spacing must be approximately equal to the scattering slab thickness.
The use of optical fibers to carry light is appropriate for the present application, especially when parallelism is used. All-fiber dividers (couplers) are available allowing for an easy separation of a main laser beam into many ports. Furthermore, the light is brought close to the medium and collecting fibers can be used allowing for a safe an efficient scanner.